Depending on how crazy you want to get, there are gadgets that will measure from on top of the roof, from the heel, or you can order a Pitch Finder for about $6.00 online. Haageducation.com sells one, Illinois Lumber & Material has another (www.ilmda.com) I'm sure you can find others. They're plastic see-through cards with the pitch marked. You stand back and line up the roof to get an approximate angle. Good for quick and dirty.